The main objective of our work has been to characterize the nature of tumor associated antigens which cross-react immunologically with alloantigens. Reticulum cell sarcoma from SJL/J mice (H-2s) were shown to exhibit H-2d allospecificities by serological, cell-mediated and biochemical analyses. The exact molecular characteristics of the cross-reactive antigens are being investigated by gel electrophoresis in two dimensions.